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The autophosphorylated, active Lck is now proximally positioned to phosphorylate specific tyrosine residues within ITAMs (immunoreceptor tyrosine-based activation motifs) located within the CD3 and the TCR zeta signaling chains of the TCR. ITAMs consist of evolutionarily conserved amino-acid sequence motifs of D/ExYxxLx(6-8)YxxL. Both the tyrosine residues in the motif are phosphorylated by Lck and the TCR complex include 10 ITAMs with one ITAM in each of the CD3 chains including the three tandem ITAMs in each zeta chains.